Milky Way consumes stars, galaxies

The Milky Way is slowly expanding its domain by ingesting neighboring galaxies and stars, according to a recent study conducted by Yale astronomers.

Astronomy student Ana Bonaca GRD ’16 began researching the integration of star clusters neighboring the Milky Way for her first-year project in 2010. Her findings were compiled in a paper soon to be published in the peer-reviewed Astrophysical Journal Letters. Bonaca used Yale’s access to the Sloan Digital Sky Survey database to create maps of outer space. These maps allowed her to locate an extremely narrow stream of stars on the periphery of the Milky Way, which Bonaca said showed the galaxy is incorporating bordering star clusters into its domain.

“These stars are likely to be part of destroyed galaxies,” Bonaca said.

Researchers determined that the stream of stars constituted a star cluster as opposed to a dwarf galaxy, which has more mass. Star clusters, unlike dwarf galaxies, are not held together by dark matter and are less broad. The team used a mass filter technique to determine the identity of the discovered system, said Nitya Kallivayalil, a postdoctoral fellow in physics who co-authored the paper.

Bonaca said the find was especially significant as it was the first star cluster found in a region of space called the southern galactic hemisphere. Previous star clusters have been found in the northern hemisphere, she added.

Bonaca credited Yale astronomy professor Marla Geha with giving her the idea for the project. Geha and Bonaca collaborated with Kallivayalil to write the paper on the star cluster discovery.

“The thing about being at Yale is having faculty with great research ideas,” she said. “It isn’t equipment that’s crucial, but people and the environment.”

Geha was awarded the Sloan Foundation’s Alfred P. Sloan Research Fellowship in 2010, which she said was a “generous grant” that helped support Bonaca over the summer. In an email to the News, Geha also credited the Foundation with providing data through its Digital Sky Service, which she said had “fundamentally changed astronomy.” Yale’s access to the digital sky survey will be used to make further measurements in the local galaxy group of the Milky Way and Andromeda, Kallivayalil said. The research team also seeks to conduct follow-up observations of the stream using the University’s access to the Keck Telescope, one of the world’s largest optical and infrared telescopes.

“Measuring the velocity of stars in this stream allows us to determine the orbits, and in turn determine the distribution of mass in the Milky Way,” Geha said.

The results of the research could be used to facilitate extremely good probes of the galaxy’s mass distribution at large distances and to measure the velocities of individual stars in the stream, Bonaca said. Discovering the “model for destruction” of galaxies consumed by the Milky Way could yield information about the “halo of the Milky Way,” a portion of the galaxy made up of destroyed clusters, she added.

This spring, the research team will put forward the proposal for measuring the velocity of the stars in the newly discovered cluster, Bonaca said.